Chemistry Reference
In-Depth Information
Fig. 12 IR-MPD spectra
for the even-sized gold
cluster complexes Au
n
O
2
.
All show an absorption in
the energy range which
corresponds to a superoxo
stretch. Two traces are
given for Au
8
O
2
demonstrating the limiting
cases of the spectra
observed by varying
the source conditions.
From [
11
]
Evidence for this binding mechanism comes from multiple sources. Several
anion PES spectra have been recorded for these clusters and provide two crucial
observations: Firstly, a vibrational progression is seen in the spectra of Au
2
O
2
,
Au
4
O
2
and Au
6
O
2
with stretching frequencies ranging between 1226 and
1443 cm
1
[
14
,
15
,
94
]. Such a vibrational frequency corresponds to a partially
activated O
2
, somewhere between physisorbed oxygen and a superoxo species.
Secondly, when the experiment is performed at cryogenic temperatures, it is
possible to form the oxygen complexes with the odd-sized clusters. The resulting
spectra, however, are almost identical to that of the parent bare gold anion,
indicating that no electronic structure changes have occurred in the complexation,
i.e. a purely van der Waals interaction [
15
].
Another PES study [
95
], which focused on the larger anions (Au
6
-Au
20
),
found a transition in the binding geometry of the superoxo moiety from being
bound to a single Au atom (
ʼ
2
) with a critical size for the
change at Au
8
. This conclusion, however, is based on the supporting DFT
calculations as no vibrational progressions were observed in the PES.
Direct evidence for the presence of superoxo species comes from IR-MPD
spectroscopy. The IR-MPD spectra for the dioxygen complexes of the anionic
species Au
4
-Au
20
clearly show absorption between 1,050 and 1,100 cm
1
(Fig.
12
)[
11
]. This unambiguously demonstrates the presence of a superoxo moiety
bound to the gold cluster as no other vibrational fundamentals are expected at these
frequencies. The observed frequencies, however, disagree with the vibrational
progressions recorded from anion PES. This is most likely due to the nature of
PES which is sensitive to both the anionic and the neutral states. The vibrational
progressions in the anion PES are now thought to instead arise from
photodetachment to an electronically excited state of the neutral cluster which
corresponds to the reaction of the gold cluster with singlet oxygen, rather than
being representative of the superoxo state [
15
].
Two very different experiments, anion PES [
95
] and IR-MPD [
11
], permit
an experimental cross validation for the special behaviour observed for the
Au
8
O
2
complex. Both studies find that the observed experimental spectra are
very sensitive on the exact source conditions employed. In the reproduced
1
) to two Au atoms (
ʼ
